/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.hadoop.hbase.client;

import static java.util.stream.Collectors.toList;
import static org.apache.hadoop.hbase.HConstants.EMPTY_END_ROW;
import static org.apache.hadoop.hbase.HConstants.EMPTY_START_ROW;
import static org.apache.hadoop.hbase.util.FutureUtils.addListener;

import java.io.IOException;
import java.lang.reflect.UndeclaredThrowableException;
import java.net.InetAddress;
import java.net.UnknownHostException;
import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.hbase.Cell;
import org.apache.hadoop.hbase.CellComparator;
import org.apache.hadoop.hbase.HConstants;
import org.apache.hadoop.hbase.PrivateCellUtil;
import org.apache.hadoop.hbase.RegionLocations;
import org.apache.hadoop.hbase.ServerName;
import org.apache.hadoop.hbase.TableName;
import org.apache.hadoop.hbase.client.metrics.ScanMetrics;
import org.apache.hadoop.hbase.ipc.HBaseRpcController;
import org.apache.hadoop.hbase.security.User;
import org.apache.hadoop.hbase.security.UserProvider;
import org.apache.hadoop.hbase.util.Bytes;
import org.apache.hadoop.hbase.util.ReflectionUtils;
import org.apache.hadoop.ipc.RemoteException;
import org.apache.hadoop.net.DNS;
import org.apache.yetus.audience.InterfaceAudience;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import org.apache.hbase.thirdparty.com.google.common.annotations.VisibleForTesting;
import org.apache.hbase.thirdparty.com.google.common.base.Preconditions;
import org.apache.hbase.thirdparty.com.google.protobuf.ServiceException;
import org.apache.hbase.thirdparty.io.netty.util.Timer;

import org.apache.hadoop.hbase.shaded.protobuf.ProtobufUtil;
import org.apache.hadoop.hbase.shaded.protobuf.ResponseConverter;
import org.apache.hadoop.hbase.shaded.protobuf.generated.AdminProtos.AdminService;
import org.apache.hadoop.hbase.shaded.protobuf.generated.ClientProtos;
import org.apache.hadoop.hbase.shaded.protobuf.generated.ClientProtos.ClientService;
import org.apache.hadoop.hbase.shaded.protobuf.generated.ClientProtos.ScanResponse;
import org.apache.hadoop.hbase.shaded.protobuf.generated.MasterProtos.MasterService;

/**
 * Utility used by client connections.
 */
@InterfaceAudience.Private
public final class ConnectionUtils {

    private static final Logger LOG = LoggerFactory.getLogger(ConnectionUtils.class);

    private ConnectionUtils() {
    }

    /**
     * Calculate pause time. Built on {@link HConstants#RETRY_BACKOFF}.
     *
     * @param pause time to pause
     * @param tries amount of tries
     * @return How long to wait after <code>tries</code> retries
     */
    public static long getPauseTime(final long pause, final int tries) {
        int ntries = tries;
        if(ntries >= HConstants.RETRY_BACKOFF.length) {
            ntries = HConstants.RETRY_BACKOFF.length - 1;
        }
        if(ntries < 0) {
            ntries = 0;
        }

        long normalPause = pause * HConstants.RETRY_BACKOFF[ntries];
        // 1% possible jitter
        long jitter = (long) (normalPause * ThreadLocalRandom.current().nextFloat() * 0.01f);
        return normalPause + jitter;
    }

    /**
     * @param conn The connection for which to replace the generator.
     * @param cnm  Replaces the nonce generator used, for testing.
     * @return old nonce generator.
     */
    public static NonceGenerator injectNonceGeneratorForTesting(ClusterConnection conn, NonceGenerator cnm) {
        return ConnectionImplementation.injectNonceGeneratorForTesting(conn, cnm);
    }

    /**
     * Changes the configuration to set the number of retries needed when using Connection internally,
     * e.g. for updating catalog tables, etc. Call this method before we create any Connections.
     *
     * @param c   The Configuration instance to set the retries into.
     * @param log Used to log what we set in here.
     */
    public static void setServerSideHConnectionRetriesConfig(final Configuration c, final String sn, final Logger log) {
        // TODO: Fix this. Not all connections from server side should have 10 times the retries.
        int hcRetries = c.getInt(HConstants.HBASE_CLIENT_RETRIES_NUMBER, HConstants.DEFAULT_HBASE_CLIENT_RETRIES_NUMBER);
        // Go big. Multiply by 10. If we can't get to meta after this many retries
        // then something seriously wrong.
        int serversideMultiplier = c
                .getInt(HConstants.HBASE_CLIENT_SERVERSIDE_RETRIES_MULTIPLIER, HConstants.DEFAULT_HBASE_CLIENT_SERVERSIDE_RETRIES_MULTIPLIER);
        int retries = hcRetries * serversideMultiplier;
        c.setInt(HConstants.HBASE_CLIENT_RETRIES_NUMBER, retries);
        log.info(sn + " server-side Connection retries=" + retries);
    }

    /**
     * A ClusterConnection that will short-circuit RPC making direct invocations against the localhost
     * if the invocation target is 'this' server; save on network and protobuf invocations.
     */
    // TODO This has to still do PB marshalling/unmarshalling stuff. Check how/whether we can avoid.
    @VisibleForTesting // Class is visible so can assert we are short-circuiting when expected.
    public static class ShortCircuitingClusterConnection extends ConnectionImplementation {
        private final ServerName serverName;
        private final AdminService.BlockingInterface localHostAdmin;
        private final ClientService.BlockingInterface localHostClient;

        private ShortCircuitingClusterConnection(Configuration conf, ExecutorService pool, User user, ServerName serverName,
                AdminService.BlockingInterface admin, ClientService.BlockingInterface client) throws IOException {

            /********
             * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
             *   注释： 初始化链接
             */
            super(conf, pool, user);
            this.serverName = serverName;
            this.localHostAdmin = admin;
            this.localHostClient = client;
        }

        @Override
        public AdminService.BlockingInterface getAdmin(ServerName sn) throws IOException {
            return serverName.equals(sn) ? this.localHostAdmin : super.getAdmin(sn);
        }

        @Override
        public ClientService.BlockingInterface getClient(ServerName sn) throws IOException {
            return serverName.equals(sn) ? this.localHostClient : super.getClient(sn);
        }

        @Override
        public MasterKeepAliveConnection getMaster() throws IOException {
            if(this.localHostClient instanceof MasterService.BlockingInterface) {
                return new ShortCircuitMasterConnection((MasterService.BlockingInterface) this.localHostClient);
            }
            return super.getMaster();
        }
    }

    /**
     * Creates a short-circuit connection that can bypass the RPC layer (serialization,
     * deserialization, networking, etc..) when talking to a local server.
     *
     * @param conf       the current configuration
     * @param pool       the thread pool to use for batch operations
     * @param user       the user the connection is for
     * @param serverName the local server name
     * @param admin      the admin interface of the local server
     * @param client     the client interface of the local server
     * @return an short-circuit connection.
     * @throws IOException if IO failure occurred
     */
    public static ClusterConnection createShortCircuitConnection(final Configuration conf, ExecutorService pool, User user,
            final ServerName serverName, final AdminService.BlockingInterface admin,
            final ClientService.BlockingInterface client) throws IOException {
        if(user == null) {
            user = UserProvider.instantiate(conf).getCurrent();
        }

        /********
         * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
         *   注释：
         */
        return new ShortCircuitingClusterConnection(conf, pool, user, serverName, admin, client);
    }

    /**
     * Setup the connection class, so that it will not depend on master being online. Used for testing
     *
     * @param conf configuration to set
     */
    @VisibleForTesting
    public static void setupMasterlessConnection(Configuration conf) {
        conf.set(ClusterConnection.HBASE_CLIENT_CONNECTION_IMPL, MasterlessConnection.class.getName());
    }

    /**
     * Some tests shut down the master. But table availability is a master RPC which is performed on
     * region re-lookups.
     */
    static class MasterlessConnection extends ConnectionImplementation {
        MasterlessConnection(Configuration conf, ExecutorService pool, User user) throws IOException {
            super(conf, pool, user);
        }

        @Override
        public boolean isTableDisabled(TableName tableName) throws IOException {
            // treat all tables as enabled
            return false;
        }
    }

    /**
     * Return retires + 1. The returned value will be in range [1, Integer.MAX_VALUE].
     */
    static int retries2Attempts(int retries) {
        return Math.max(1, retries == Integer.MAX_VALUE ? Integer.MAX_VALUE : retries + 1);
    }

    /**
     * Get a unique key for the rpc stub to the given server.
     */
    static String getStubKey(String serviceName, ServerName serverName, boolean hostnameCanChange) {
        // Sometimes, servers go down and they come back up with the same hostname but a different
        // IP address. Force a resolution of the rsHostname by trying to instantiate an
        // InetSocketAddress, and this way we will rightfully get a new stubKey.
        // Also, include the hostname in the key so as to take care of those cases where the
        // DNS name is different but IP address remains the same.
        String hostname = serverName.getHostname();
        int port = serverName.getPort();
        if(hostnameCanChange) {
            try {
                InetAddress ip = InetAddress.getByName(hostname);
                return serviceName + "@" + hostname + "-" + ip.getHostAddress() + ":" + port;
            } catch(UnknownHostException e) {
                LOG.warn("Can not resolve " + hostname + ", please check your network", e);
            }
        }
        return serviceName + "@" + hostname + ":" + port;
    }

    static void checkHasFamilies(Mutation mutation) {
        Preconditions.checkArgument(mutation.numFamilies() > 0, "Invalid arguments to %s, zero columns specified", mutation.toString());
    }

    /**
     * Dummy nonce generator for disabled nonces.
     */
    static final NonceGenerator NO_NONCE_GENERATOR = new NonceGenerator() {

        @Override
        public long newNonce() {
            return HConstants.NO_NONCE;
        }

        @Override
        public long getNonceGroup() {
            return HConstants.NO_NONCE;
        }
    };

    // A byte array in which all elements are the max byte, and it is used to
    // construct closest front row
    static final byte[] MAX_BYTE_ARRAY = Bytes.createMaxByteArray(9);

    /**
     * Create the closest row after the specified row
     */
    static byte[] createClosestRowAfter(byte[] row) {
        return Arrays.copyOf(row, row.length + 1);
    }

    /**
     * Create a row before the specified row and very close to the specified row.
     */
    static byte[] createCloseRowBefore(byte[] row) {
        if(row.length == 0) {
            return MAX_BYTE_ARRAY;
        }
        if(row[row.length - 1] == 0) {
            return Arrays.copyOf(row, row.length - 1);
        } else {
            byte[] nextRow = new byte[row.length + MAX_BYTE_ARRAY.length];
            System.arraycopy(row, 0, nextRow, 0, row.length - 1);
            nextRow[row.length - 1] = (byte) ((row[row.length - 1] & 0xFF) - 1);
            System.arraycopy(MAX_BYTE_ARRAY, 0, nextRow, row.length, MAX_BYTE_ARRAY.length);
            return nextRow;
        }
    }

    static boolean isEmptyStartRow(byte[] row) {
        return Bytes.equals(row, EMPTY_START_ROW);
    }

    static boolean isEmptyStopRow(byte[] row) {
        return Bytes.equals(row, EMPTY_END_ROW);
    }

    static void resetController(HBaseRpcController controller, long timeoutNs, int priority) {
        controller.reset();
        if(timeoutNs >= 0) {
            controller.setCallTimeout((int) Math.min(Integer.MAX_VALUE, TimeUnit.NANOSECONDS.toMillis(timeoutNs)));
        }
        controller.setPriority(priority);
    }

    static Throwable translateException(Throwable t) {
        if(t instanceof UndeclaredThrowableException && t.getCause() != null) {
            t = t.getCause();
        }
        if(t instanceof RemoteException) {
            t = ((RemoteException) t).unwrapRemoteException();
        }
        if(t instanceof ServiceException && t.getCause() != null) {
            t = translateException(t.getCause());
        }
        return t;
    }

    static long calcEstimatedSize(Result rs) {
        long estimatedHeapSizeOfResult = 0;
        // We don't make Iterator here
        for(Cell cell : rs.rawCells()) {
            estimatedHeapSizeOfResult += cell.heapSize();
        }
        return estimatedHeapSizeOfResult;
    }

    static Result filterCells(Result result, Cell keepCellsAfter) {
        if(keepCellsAfter == null) {
            // do not need to filter
            return result;
        }
        // not the same row
        if(!PrivateCellUtil.matchingRows(keepCellsAfter, result.getRow(), 0, result.getRow().length)) {
            return result;
        }
        Cell[] rawCells = result.rawCells();
        int index = Arrays.binarySearch(rawCells, keepCellsAfter, CellComparator.getInstance()::compareWithoutRow);
        if(index < 0) {
            index = -index - 1;
        } else {
            index++;
        }
        if(index == 0) {
            return result;
        }
        if(index == rawCells.length) {
            return null;
        }
        return Result.create(Arrays.copyOfRange(rawCells, index, rawCells.length), null, result.isStale(), result.mayHaveMoreCellsInRow());
    }

    // Add a delta to avoid timeout immediately after a retry sleeping.
    static final long SLEEP_DELTA_NS = TimeUnit.MILLISECONDS.toNanos(1);

    static Get toCheckExistenceOnly(Get get) {
        if(get.isCheckExistenceOnly()) {
            return get;
        }
        return ReflectionUtils.newInstance(get.getClass(), get).setCheckExistenceOnly(true);
    }

    static List<Get> toCheckExistenceOnly(List<Get> gets) {
        return gets.stream().map(ConnectionUtils::toCheckExistenceOnly).collect(toList());
    }

    static RegionLocateType getLocateType(Scan scan) {
        if(scan.isReversed()) {
            if(isEmptyStartRow(scan.getStartRow())) {
                return RegionLocateType.BEFORE;
            } else {
                return scan.includeStartRow() ? RegionLocateType.CURRENT : RegionLocateType.BEFORE;
            }
        } else {
            return scan.includeStartRow() ? RegionLocateType.CURRENT : RegionLocateType.AFTER;
        }
    }

    static boolean noMoreResultsForScan(Scan scan, RegionInfo info) {
        if(isEmptyStopRow(info.getEndKey())) {
            return true;
        }
        if(isEmptyStopRow(scan.getStopRow())) {
            return false;
        }
        int c = Bytes.compareTo(info.getEndKey(), scan.getStopRow());
        // 1. if our stop row is less than the endKey of the region
        // 2. if our stop row is equal to the endKey of the region and we do not include the stop row
        // for scan.
        return c > 0 || (c == 0 && !scan.includeStopRow());
    }

    static boolean noMoreResultsForReverseScan(Scan scan, RegionInfo info) {
        if(isEmptyStartRow(info.getStartKey())) {
            return true;
        }
        if(isEmptyStopRow(scan.getStopRow())) {
            return false;
        }
        // no need to test the inclusive of the stop row as the start key of a region is included in
        // the region.
        return Bytes.compareTo(info.getStartKey(), scan.getStopRow()) <= 0;
    }

    static <T> CompletableFuture<List<T>> allOf(List<CompletableFuture<T>> futures) {
        return CompletableFuture.allOf(futures.toArray(new CompletableFuture[0]))
                .thenApply(v -> futures.stream().map(f -> f.getNow(null)).collect(toList()));
    }

    public static ScanResultCache createScanResultCache(Scan scan) {
        if(scan.getAllowPartialResults()) {
            return new AllowPartialScanResultCache();
        } else if(scan.getBatch() > 0) {
            return new BatchScanResultCache(scan.getBatch());
        } else {
            return new CompleteScanResultCache();
        }
    }

    private static final String MY_ADDRESS = getMyAddress();

    private static String getMyAddress() {
        try {
            return DNS.getDefaultHost("default", "default");
        } catch(UnknownHostException uhe) {
            LOG.error("cannot determine my address", uhe);
            return null;
        }
    }

    static boolean isRemote(String host) {
        return !host.equalsIgnoreCase(MY_ADDRESS);
    }

    static void incRPCCallsMetrics(ScanMetrics scanMetrics, boolean isRegionServerRemote) {
        if(scanMetrics == null) {
            return;
        }
        scanMetrics.countOfRPCcalls.incrementAndGet();
        if(isRegionServerRemote) {
            scanMetrics.countOfRemoteRPCcalls.incrementAndGet();
        }
    }

    static void incRPCRetriesMetrics(ScanMetrics scanMetrics, boolean isRegionServerRemote) {
        if(scanMetrics == null) {
            return;
        }
        scanMetrics.countOfRPCRetries.incrementAndGet();
        if(isRegionServerRemote) {
            scanMetrics.countOfRemoteRPCRetries.incrementAndGet();
        }
    }

    static void updateResultsMetrics(ScanMetrics scanMetrics, Result[] rrs, boolean isRegionServerRemote) {
        if(scanMetrics == null || rrs == null || rrs.length == 0) {
            return;
        }
        long resultSize = 0;
        for(Result rr : rrs) {
            for(Cell cell : rr.rawCells()) {
                resultSize += PrivateCellUtil.estimatedSerializedSizeOf(cell);
            }
        }
        scanMetrics.countOfBytesInResults.addAndGet(resultSize);
        if(isRegionServerRemote) {
            scanMetrics.countOfBytesInRemoteResults.addAndGet(resultSize);
        }
    }

    /**
     * Use the scan metrics returned by the server to add to the identically named counters in the
     * client side metrics. If a counter does not exist with the same name as the server side metric,
     * the attempt to increase the counter will fail.
     */
    static void updateServerSideMetrics(ScanMetrics scanMetrics, ScanResponse response) {
        if(scanMetrics == null || response == null || !response.hasScanMetrics()) {
            return;
        }
        ResponseConverter.getScanMetrics(response).forEach(scanMetrics::addToCounter);
    }

    static void incRegionCountMetrics(ScanMetrics scanMetrics) {
        if(scanMetrics == null) {
            return;
        }
        scanMetrics.countOfRegions.incrementAndGet();
    }

    /**
     * Connect the two futures, if the src future is done, then mark the dst future as done. And if
     * the dst future is done, then cancel the src future. This is used for timeline consistent read.
     * <p/>
     * Pass empty metrics if you want to link the primary future and the dst future so we will not
     * increase the hedge read related metrics.
     */
    private static <T> void connect(CompletableFuture<T> srcFuture, CompletableFuture<T> dstFuture, Optional<MetricsConnection> metrics) {
        addListener(srcFuture, (r, e) -> {
            if(e != null) {
                dstFuture.completeExceptionally(e);
            } else {
                if(dstFuture.complete(r)) {
                    metrics.ifPresent(MetricsConnection::incrHedgedReadWin);
                }
            }
        });
        // The cancellation may be a dummy one as the dstFuture may be completed by this srcFuture.
        // Notice that this is a bit tricky, as the execution chain maybe 'complete src -> complete dst
        // -> cancel src', for now it seems to be fine, as the will use CAS to set the result first in
        // CompletableFuture. If later this causes problems, we could use whenCompleteAsync to break the
        // tie.
        addListener(dstFuture, (r, e) -> srcFuture.cancel(false));
    }

    private static <T> void sendRequestsToSecondaryReplicas(Function<Integer, CompletableFuture<T>> requestReplica, RegionLocations locs,
            CompletableFuture<T> future, Optional<MetricsConnection> metrics) {
        if(future.isDone()) {
            // do not send requests to secondary replicas if the future is done, i.e, the primary request
            // has already been finished.
            return;
        }
        for(int replicaId = 1, n = locs.size(); replicaId < n; replicaId++) {
            CompletableFuture<T> secondaryFuture = requestReplica.apply(replicaId);
            metrics.ifPresent(MetricsConnection::incrHedgedReadOps);
            connect(secondaryFuture, future, metrics);
        }
    }

    static <T> CompletableFuture<T> timelineConsistentRead(AsyncRegionLocator locator, TableName tableName, Query query, byte[] row,
            RegionLocateType locateType, Function<Integer, CompletableFuture<T>> requestReplica, long rpcTimeoutNs, long primaryCallTimeoutNs,
            Timer retryTimer, Optional<MetricsConnection> metrics) {
        if(query.getConsistency() != Consistency.TIMELINE) {
            return requestReplica.apply(RegionReplicaUtil.DEFAULT_REPLICA_ID);
        }
        // user specifies a replica id explicitly, just send request to the specific replica
        if(query.getReplicaId() >= 0) {
            return requestReplica.apply(query.getReplicaId());
        }
        // Timeline consistent read, where we may send requests to other region replicas
        CompletableFuture<T> primaryFuture = requestReplica.apply(RegionReplicaUtil.DEFAULT_REPLICA_ID);
        CompletableFuture<T> future = new CompletableFuture<>();
        connect(primaryFuture, future, Optional.empty());
        long startNs = System.nanoTime();
        // after the getRegionLocations, all the locations for the replicas of this region should have
        // been cached, so it is not big deal to locate them again when actually sending requests to
        // these replicas.
        addListener(locator.getRegionLocations(tableName, row, locateType, false, rpcTimeoutNs), (locs, error) -> {
            if(error != null) {
                LOG.warn("Failed to locate all the replicas for table={}, row='{}', locateType={}" + " give up timeline consistent read", tableName,
                        Bytes.toStringBinary(row), locateType, error);
                return;
            }
            if(locs.size() <= 1) {
                LOG.warn("There are no secondary replicas for region {}, give up timeline consistent read",
                        locs.getDefaultRegionLocation().getRegion());
                return;
            }
            long delayNs = primaryCallTimeoutNs - (System.nanoTime() - startNs);
            if(delayNs <= 0) {
                sendRequestsToSecondaryReplicas(requestReplica, locs, future, metrics);
            } else {
                retryTimer
                        .newTimeout(timeout -> sendRequestsToSecondaryReplicas(requestReplica, locs, future, metrics), delayNs, TimeUnit.NANOSECONDS);
            }
        });
        return future;
    }

    /********
     * TODO_MA 马中华 https://blog.csdn.net/zhongqi2513
     *   注释： 客户端在提交RPC请求之前会进行一次校验，校验内容为
     *   1、是否有列, put.isEmpty()。
     *   2、put对象大小是否超过设定值（默认最大值为10485760（10M），客户端配置参数为MAX_KEYVALUE_SIZE_KEY = "hbase.client.keyvalue.maxsize"）
     *   校验不通过时抛出异常 throw new IllegalArgumentException("No columns to insert" ）
     *   或者 throw new IllegalArgumentException("KeyValue size too large")
     */
    // validate for well-formedness
    static void validatePut(Put put, int maxKeyValueSize) throws IllegalArgumentException {
        if(put.isEmpty()) {
            throw new IllegalArgumentException("No columns to insert");
        }
        if(maxKeyValueSize > 0) {
            for(List<Cell> list : put.getFamilyCellMap().values()) {
                for(Cell cell : list) {
                    if(cell.getSerializedSize() > maxKeyValueSize) {
                        throw new IllegalArgumentException("KeyValue size too large");
                    }
                }
            }
        }
    }

    /**
     * Select the priority for the rpc call.
     * <p/>
     * The rules are:
     * <ol>
     * <li>If user set a priority explicitly, then just use it.</li>
     * <li>For system table, use {@link HConstants#SYSTEMTABLE_QOS}.</li>
     * <li>For other tables, use {@link HConstants#NORMAL_QOS}.</li>
     * </ol>
     *
     * @param priority  the priority set by user, can be {@link HConstants#PRIORITY_UNSET}.
     * @param tableName the table we operate on
     */
    static int calcPriority(int priority, TableName tableName) {
        if(priority != HConstants.PRIORITY_UNSET) {
            return priority;
        } else {
            return getPriority(tableName);
        }
    }

    static int getPriority(TableName tableName) {
        if(tableName.isSystemTable()) {
            return HConstants.SYSTEMTABLE_QOS;
        } else {
            return HConstants.NORMAL_QOS;
        }
    }

    static <T> CompletableFuture<T> getOrFetch(AtomicReference<T> cacheRef, AtomicReference<CompletableFuture<T>> futureRef, boolean reload,
            Supplier<CompletableFuture<T>> fetch, Predicate<T> validator, String type) {
        for(; ; ) {
            if(!reload) {
                T value = cacheRef.get();
                if(value != null && validator.test(value)) {
                    return CompletableFuture.completedFuture(value);
                }
            }
            LOG.trace("{} cache is null, try fetching from registry", type);
            if(futureRef.compareAndSet(null, new CompletableFuture<>())) {
                LOG.debug("Start fetching{} from registry", type);
                CompletableFuture<T> future = futureRef.get();
                addListener(fetch.get(), (value, error) -> {
                    if(error != null) {
                        LOG.debug("Failed to fetch {} from registry", type, error);
                        futureRef.getAndSet(null).completeExceptionally(error);
                        return;
                    }
                    LOG.debug("The fetched {} is {}", type, value);
                    // Here we update cache before reset future, so it is possible that someone can get a
                    // stale value. Consider this:
                    // 1. update cacheRef
                    // 2. someone clears the cache and relocates again
                    // 3. the futureRef is not null so the old future is used.
                    // 4. we clear futureRef and complete the future in it with the value being
                    // cleared in step 2.
                    // But we do not think it is a big deal as it rarely happens, and even if it happens, the
                    // caller will retry again later, no correctness problems.
                    cacheRef.set(value);
                    futureRef.set(null);
                    future.complete(value);
                });
                return future;
            } else {
                CompletableFuture<T> future = futureRef.get();
                if(future != null) {
                    return future;
                }
            }
        }
    }

    static void updateStats(Optional<ServerStatisticTracker> optStats, Optional<MetricsConnection> optMetrics, ServerName serverName,
            MultiResponse resp) {
        if(!optStats.isPresent() && !optMetrics.isPresent()) {
            // ServerStatisticTracker and MetricsConnection are both not present, just return
            return;
        }
        resp.getResults().forEach((regionName, regionResult) -> {
            ClientProtos.RegionLoadStats stat = regionResult.getStat();
            if(stat == null) {
                LOG.error("No ClientProtos.RegionLoadStats found for server={}, region={}", serverName, Bytes.toStringBinary(regionName));
                return;
            }
            RegionLoadStats regionLoadStats = ProtobufUtil.createRegionLoadStats(stat);
            optStats.ifPresent(stats -> ResultStatsUtil.updateStats(stats, serverName, regionName, regionLoadStats));
            optMetrics.ifPresent(metrics -> ResultStatsUtil.updateStats(metrics, serverName, regionName, regionLoadStats));
        });
    }
}
